This is pretty close to the start of a great trilogy called Empire From The Ashes. An astronaut using ground-penetrating radar on a lunar mission discovers the Moon is composed mostly of empty space and ultrahard alloys, i.e. not a ball of stone like we think. Before he can tell anyone, he is whisked miles beneath the surface, where an AI tells him the Moon is actually a camouflaged Death Star and he is the new emperor of mankind.
